Rotatable wave guide joint



Patented Dec. 26, 1950 AndrewAlf'ord; Cambridge, Mass, assignor to the Unitedgtatesof A'merica asv represented bathe Secretary of War Applicationnbr'i a 194sf's'elifi No. 660,602

r .lGlaimq (Cl. 178-A4h 'iHIhiS invention'relates to electrical apparatus, and; more particularly;itrelates to: a radio free quency rotating-joint, -7.;.- Intransmissionssystems;:forv conducting radio. frequency :energy i from one point etc another throughwave. guides;..the -need f-requent1y arises fonmecha nieally rotatablejoints whichperrni't Y energy; 'Such'a's energ in the "microwave range of frequencies," i. "e'.;" havingwave lengths"of -the:

portionsof -tlier-transm-ission:system .to beImoVed rotationally. with respectto other portions while at .the'i same time the transmission of 5 radio. ire} quency energy isuninterruptei V l Imthe prior art,.onetypical.meansfor meeting.

this need hasibeen. apparatus including "round-,.

wave-guide portions mechaniallyrotatable' with respect toea'ch other. Round waveguide lends itself to thisxus'e because loflth'el axially symmetrical fiel'd' configuration" within it: However; there are certain drawbacks t'oiithis typeof apparatus-.1 Forlone 'thing itis rathercritic'al asto' frequency andv inay constitute the pass band" width-limiting elementin"'"a' system" otherwise capable-of broad "-band-'-transmissioni It follows,

that excessive-- refiectio'n will occnrfor relatively.

small variations in operating frequency, that is, forf'variationsbeyond'this narrow pass band,

win.be adverselvfaitecred. a

and that standing wave ratios of su'ch systems.

Itis an objectof' this invention provide a rotatable joint which. is capable of operation over relati'vely;broadjfrequency band. a It is also an object of this iinvntionithat this rotatable joint havealow"coeflicient'of'reflection: i

Other objects, features and advantages of this invention will suggest themselves"to"those skilled in the art and will -become'f'apparentfrom the following description of the invention taken in connection "with the accompanying drawingin' which? I i .l

Fig. 1 isa side view of. one embodi'ment'of this invention; This; embodiment. consists of two end';.th1-ough a; transition portion to tional rectangular wave guide-5:. .Radi'o 'irequency' order ofii'centiinetersY "isiisupplied to the -device guide. rectangular, applied to-"the cross section of these wave guide portions, is intended to include an effective rectangular wave guide" section s'nch a's airelliptically shaped sectiomwhichrmay :-be pro due-ea by' rounding *theicorner of areetangular' shaped section." The cross" sectional"dimensions of' the wave guidesecti'on' are-preferably lajrgr' than the corresponding*dimensionsof the waveguide: Also; the wave guide section is "singularly displaced aboutthe longitudinaraxisofthe radio frequency horn with reference tothe'waveguide so thatthe planes of 'thewave' guide section are at substantially anangle of to the planes of the waveguide. A tapered. portion. is usedto effect the transition between the wave guide secmen and the wave 'g'iiide. Thistpreferably ornprises. eight"triangularlyishaped pieces...rfi"

1. The' direction i of .the electrical'.Xu'actors.v or. thev energy. wavesi'amving atone Jena of shamans tion;.iportion is. parallellto ona of thesides. of the" wave guide. While traveling along'thetran sition portion.theielectricalzvector is directed: along the diagonal of the cross sectionof the-aware. guide sectiome Bythe-rti-me l-the :wave. artiveaat.- the" junction oilthe transition .-.portion and the wave Plane; 1to, circular: polarizationtransformers placed with corresponding lends face toface.

[Fig.2 .is a cross-sectional drawingof a broken away portion of. the'device shown. inFig. 1 showin'g ias an added element an. RF choke.

.--,'General1y, this; invention-consists-of .a 00111:;

bination of two radio irequencyhorns yeachof which is capable oftransformi-ng plane-polarized energy :into, circularly polarized energy and vice.

versa.= This transformer :is. -described*in 'my copending application; --.-S.erial\ o. 1644,175; filed Jannary..25,.- 19461;. These;horns.- consist of a trace,

tangular wave guide section joined axially with a tapered horn outlet at one end and. at the other guide section, the electrical vector. is effectively resolved into two zmutuallywperpendicular Iconiponentsiparalleli to the sides ofathe .wave guide; sect-ion. i-By making thevcross sectional 'dimena; sions both-greater than'a-halfwave-:lengthof the energy being; transmitted it= iis= possib1e for the two component" wavesatmbepropagatedalonglthe length rof thezwave -guide:-secti on;:-. l

"aBethzcomp'onent"wavesarei o :transverse elm-,1. trical type. and each'abehavesifin much the:sz'iniev m'anner'i'. as a xtransversel electrical wave zin a convtenti'bnahzwave iguidea \;-':'Ih'e1 phasevelocities; at which the two-component waves areipropagat-ic ing'are" different fromione another when thewa've guideidimensionsiare' unequali Because'sof ithis difference :in \velocities; there-. tisr a progressiv'elik" increasing: phase difference between:thestwo' elece tric svectors iiwhiclrzare at righte angles toi-iaeach other in space. After the two component waves havez ipropag atede nverl aesuitabl'el distance, the

time phase distance between the two electric vectors becomes equal to so that these vectors:

3 are not only'in space quadrature but also in time quadrature thereby resulting in circularly polarized energy waves.

This is the case at the beginning of the tapered horn. The aperture of the tapered horn may be made approximately square and the flare is usually fairly short, so that the difference in phase does not continue to increase within the horn. The result is that the tapered horn acts merely as a transformer or as an impedance matching section between the wave guide section and space-or a similar horn reducing the reflection at the end of the wave guide section to a small value. When the flare is made gradual, further small phase differentiation may occur within the tapered horn. This, however, may be allowed for by making the length of.the wave guide section somewhat shorter.

Referring now more particularly to Fig. 1, transforming horn l consists of four principal components. These components are rectangular wave guide 2, section 3 of wave guide, transition portion d for mechanically and electrically joining wave guide 2 and section 3 and tapered horn 5 which increases the characteristic impedance of section 3. The detailed structure of horn I and the physical arrangements of its principal parts are fully described above and further in my copending application referred to above.

;In operation, plane polarization rectangular wave guide 2 feeds into, or is fed by, section 3. If radio frequency energy-is fed through rectangular guide 2 in the direction of tapered horn 5, it becomes circularly polarized and the electric vector of the radio frequency energy which emerges from tapered horn 5 is a rotating one of relatively constant magnitude.

Transforming horn 3 corresponds to horn I in structure and function. It can be proven theoretically, and it has been established experimentally, that if circularly polarized radio frequency energy is fed into the end of the tapered horn portion of transforming horn 6 which is coupled to tapered horn 5 of transforming horn i, that it will be translated into plane polarized radio frequency energy at the end of horn ii which. is coupled to rectangular wave guide 2. Horn 6 will affect this transformation irrespective of the angular position of the electric vector of the input energy at any instant of time.

t can be proven theoretically and it has been established experimentally that a correspondingtransformation will be effected if energy is transferred in the opposite direction, that is from horn E into horn l.

As a result of the manner of operation of this device, rectangular wave guide 2 of transforminghorns' l and 6 may have any axial rotational relationship with respect to each other and nevertheless the radio frequency energy passing through them will be plane polarized; moreover the flow of power will be uninterrupted not only irrespective of their respective axial rotational relationship but also irrespective of any rate at which they might be moved rotationally with respect to each other, within all practical mechanical limits.

. Referring now more particularly to Fig. 2 it will be seen that a radio frequency quarter wavelength choke can readily be added to this apparatus at the point of juxtaposition between the two plane-to-circular-polarization transforming devices.

- The RF choke shown in Fig. 2 is perfectly .4 conventional in every way and it is unnecessary to describe it in detail.

Section 3 of horn I and the section of horn 6 which corresponds to it are not as frequency sensitive as are round wave guides. For this reason, the pass band of this rotating joint is governed primarily by the characteristics of other portions of the system. If transition portion 4 is made to have a length of the order of one wavelength corresponding to the lowest intended operating frequency, it will not create a sudden discontinuity which might set up substantial reflections. In the same way, tapered horn 5 of transforming horns l and 6, change the characteristic impedance of the guide to which they are attached in a gradual enough manner to avoid undesirable electrical discontinuities.

Therefore, this apparatus is capable of operation over a relatively broad frequency band and for its frequency band of operation it has a low coemcient of reflection.

It is obvious that plane-to-circular-polarization transforming devices can be employed which are not of the type used in this embodiment and described in the copending application mentioned above. In fact, any one of a variety of plane-to circular-polarization transforming device may be employed in other embodiments of this invention which follow the same basic principle of opera tion. The basic principle of this invention is that of transforming plane polarized energy into circularly polarized energy and then transforming it bacls again into a plane polarized condition. in this respect it is distinguishable from ordinary rotating joints which transform plane polarized energy into energy which, within a round wave guide, has an axially symmetrical field configuration but is not circularly polarized.

It is obvious that the mechanical details of construction of this apparatus may be selected from a wide variety of conventional techniques. It is also obvious that mechanical structure for supporting said horns and mechanical drive means for rotating them axially with respect to each other may be fashioned in accordance with any of a Wide variety of well known conventional designs. This wide variety of choices in the matter of construction of this apparatus can be exercised without departing from the spirit and scope of this invention.

It will be apparent that there may be deviations from the invention as described and claimed which still fall fairly within the spirit and scope of the invention.

What is claimed is:

A device for coup'ing linearly-polarized electromagnetic waves from a fixed to a rotatable rectangular wave guide over a broad operating frequency band comprising first and second rectangular wave guides for translating linearly polarized electromagnetic waves, one of said wave guides being fixed and the other being rotatable, first and second rectangular wave guide sections having their surfaces dispiaced 45 from the surfaces of said first and second wave guides, respectively, first and second wave guide transformer sections coupled between said first and second wave guides and said first and second wave guide sections, respectively, and shaped to transform linearly-polarized electromagnetic waves into a pair of waves linearly-polarized perpendicularly to one another, said wave guide sections having cross-sectional dimensions large enough to propagate said pair of waves polarized ANDREW ALFORD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Korman July 2, 1946 Mieher Sept. 10, 1946 Beck Mar. 4, 1947 Ring Aug. 12, 1947 Fox Mar. 23, 1948 

